Polyacetal resins, or polymeric compounds having recurring --CH.sub.2 --O-- units, are well known and widely used as typical engineering plastics.
However, most polyacetal resins are used exclusively for injection moldings and are rarely used for extrusion moldings and films. The reason for such usage is that film forming of polyacetal resins is extremely difficult.
Generally, two types of film producing processes are known, namely, a T-die process and an inflation process. The T-die process generally comprises broadening a stream of molten plastic introduced from one spot to the desired film width so as to change the stream into one having as much thickness uniformity as possible, extruding it through a slip-shaped nozzle in the form of a membrane, then cooling it into a film. The inflation process is often employed in forming a film of polyethylene. It comprises extruding molten plastic through an annular extrusion nozzle in a tubular pattern, blowing air or like fluid into the tube, thereby inflating the tube to form a tubular film. The T-die process and the inflation process have their respective advantages and disadvantages. As compared with the T-die process, the inflation process generally results in greater film productivity and is more economical. The inflation process is also suitable for thin-gage film forming, but its adaptability varies depending upon the particular resin mterial being processed As such, it cannot be said that the inflation process is applicable to all types of resins. With polyacetal resins, various technical problems exist with respect to manufacturing films there of by the inflation process. Therefore, polyacetal resin film manufacturing has not been commercialized.
By means of the present invention, however, it is now possible to manufacture polyacetal resin films by the inflation process. As a result, it possible to apply the inflation process to polyacetal resin film manufacturing by selecting a certain polyacetal resin and certain temperature conditions, as will be defined in greater detail below.